1. Field of the Invention
The present invention relates to a projection exposure apparatus and, more particularly, to a projection exposure apparatus used in a manufacturing process of a semiconductor integrated circuit.
2. Related Background Art
In recent years, in a lithography process, as an apparatus for transferring a micro-pattern onto a photosensitive substrate (a semiconductor wafer formed with a resist layer), a step-and-repeat type reduction projection exposure apparatus stepper) is often used. In a stepper of this type, since the resolution line width on a wafer has reached a value on the submicron order (about 0.5 .mu.m) as the degree of integration of semiconductor elements has been increased, a pattern of a reticle (having the same meaning as a mask) must be aligned to one shot region on the wafer with alignment precision (normally, about 1/5 the resolution line width) corresponding to the resolution.
In a projection exposure apparatus such as a stepper, in order to focus a reticle pattern on a wafer at high resolution, the chromatic aberration of a projection optical system is satisfactorily corrected for only exposure illumination light (e.g., KrF excimer laser light having a wavelength of 248 nm) in the present state. Therefore, when an alignment system for overlaying a projection image of a reticle pattern onto a shot region employs a TTR (through-the-reticle) method or a TTL (through-the-lens) method, light for illuminating an alignment mark is illumination light having a wavelength equal to or very close to that of exposure light. For this reason, the alignment illumination light is attenuated by a resist layer before it reaches a wafer mark, and light reflected by the mark (regularly reflected light, scattered light, diffracted light, and the like) is also attenuated. Thus, the wafer mark cannot be recognized with a sufficient light amount, and the detection precision of the alignment system is undesirably impaired. Furthermore, when alignment illumination light is radiated onto the wafer mark, the corresponding portion of the resist layer is exposed. When the wafer is subjected to various processes after development, the corresponding mark on the wafer is destroyed. For this reason, the wafer mark cannot be used in alignment in an overlaying exposure process of the next layer.
As disclosed in, e.g., U.S. Pat. No. 4,677,301 and U.S. Pat. No. 5,004,348, as a light source for emitting alignment illumination light, a light source (e.g., an He-Ne laser having a wavelength of 633 nm) having a wavelength different from that of exposure light is used, an alignment mark formed on a wafer or reticle is detected, and the position of the wafer or reticle is detected based on the optical information.
In a method disclosed in U.S. Pat. No. 4,677,301, a mark formed on a wafer or reticle is relatively scanned by spot light (having a slit pattern) obtained by focusing a laser beam, scattered and diffracted light components from the mark edge are photoelectrically detected, thereby detecting a mark position on the basis of the center of a photoelectric signal waveform (to be referred to as an LSA (laser step alignment) method hereinafter). In a method disclosed in U.S. Pat. No. 5,004,348, coherent laser beams (parallel light beams) are simultaneously radiated from two directions on a diffraction grating mark to form one-dimensional interference fringes, and the position of the diffraction grating mark is specified using the interference fringes. The alignment method using interference fringes includes a heterodyne method for providing a predetermined frequency difference between two laser beams radiated from two directions, and a homodyne method having no frequency difference. In the homodyne method, still interference fringes are formed to be parallel to the diffraction grating mark, and the diffraction grating mark (object) must be finely moved in its pitch direction in position detection. The position of the grating mark is obtained with reference to the interference fringes. In contrast to this, in the heterodyne method, interference fringes are moved at a high speed in their fringe direction (pitch direction) according to the frequency difference (beat frequency) between the two laser beams. The position of the grating mark cannot be obtained with reference to the interference fringes, but is obtained with reference to a time factor (phase difference) upon high-speed movement of the interference fringes.
For example, in the heterodyne method, a phase difference (within .+-.180.degree.) between a photoelectric signal (optical beat signal) detected by intensity-modulating .+-.1st-order diffracted light from the grating mark, and an optical beat signal of reference interference light, which is separately formed based on two light beams using a reference grating, is obtained, thereby detecting a position shift of the grating mark within .+-.P/4 a grating pitch P.
In the above-mentioned LSA method, or in the alignment method using the diffraction grating, in an optical system (illumination system) for guiding alignment light, a field stop is arranged at a position substantially conjugate with a wafer. The field stop is used for limiting an illumination region for illuminating only an alignment mark formed on a wafer, and is particularly used for preventing reflected light by a circuit pattern on a transfer region or another alignment mark from becoming incident on a detector as noise light.
However, in the above-mentioned prior art, alignment light is incident on a projection optical system from a direction outside the optical axis, and an astigmatism occurs due to the difference between the wavelengths of exposure light and alignment light. Assume that a field stop is arranged at a plane substantially conjugate with a wafer in an illumination system, and at a sagittal image surface (a position where an edge extending in a direction perpendicular to the sagittal direction within the visual field of the projection optical system is focused) of the projection optical system with respect to alignment light. An image pattern of the field stop on the wafer is then formed in a defocus state in a meridional direction (to be referred to as an "M direction" hereinafter) in the visual field of the projection optical system due to the astigmatism (i.e., the edge extending in the sagittal direction in the visual field of the projection optical system is formed in a defocus state). For this reason, a proper illumination field (illumination region) cannot be formed, and illumination light falling outside a region near the alignment mark on the wafer undesirably illuminates other alignment marks and a circuit pattern. In addition, stray light is produced under the influence of, e.g., inter-surface reflection between the projection optical system and the wafer, or between lens elements of the projection optical system. For this reason, light from a portion other than a predetermined alignment mark is mixed in an output from an alignment sensor as noise. For example, a case will be examined below wherein a position shift of an alignment mark on a wafer with respect to a reference grating is detected using the heterodyne method.
FIG. 12 shows the positional relationship between a visual field if of the projection optical system and alignment marks on a wafer. A plurality of alignment marks WM1 to WM4 are formed outside a rectangular pattern region
inscribing the visual field if of the projection optical system so as to be adjacent to the pattern region PA. Assume that the diffraction-grating-like alignment marks WM1 to WM4 are arranged on the wafer, so that the sagittal direction (to be referred to as the "S direction" hereinafter) in the visual field if of the projection optical system coincides with a measurement direction. More specifically, two light beams are incident from the S direction on the mark at a predetermined angle, and measurement is performed using an edge (an edge in the M direction) extending in a direction perpendicular to the S direction. Therefore, the edge of the alignment mark extends in substantially the M direction. Note that the M direction is a radiation direction having an optical axis AX in the visual field if of the projection optical system as the center, and the S direction is a direction perpendicular to the M direction. FIG. 10 pays attention to one of the plurality of alignment marks, and shows the alignment mark WM1 and an illumination region SA. In this case, a rectangular stop is arranged at the sagittal image surface located at a position substantially conjugate with the wafer, and an edge extending in a direction perpendicular to the S direction is focused on the wafer W. This edge is indicated by a bold line Ed in FIG. 11. In contrast to this, as described above, the edge extending in the direction perpendicular to the M direction is formed in a defocus state (a state indicated by a wavy line) due to the astigmatism. As a result, the illumination region cannot be clearly defined. Therefore, interference light produced from marks other than the alignment mark WM1 to be detected or a circuit pattern P is mixed in interference light to be detected as a noise light component as represented by the following equation, thus causing a phase shift. EQU .phi.=Acos(.omega.t+.phi.)+.SIGMA.Ncos(.phi.t+.phi..sub.N) (1)
(A: amplitude, .omega.: beat frequency, .phi.: phase, N: amplitude of noise component, .phi.: phase of noise component)
In this method, the phase shift is detected as an alignment shift, and as a result, an alignment error occurs.
FIG. 11 shows the phase shift state. The abscissa represents a beat signal (reference signal) component of reference interference light. A vector component of a beat signal of interference light from the mark WM1 is represented by A.sub.S, and a vector component of a beat signal of noise light is represented by N.sub.S. .phi..sub.A represents a phase difference between the beat signal (reference signal) of reference interference light, and the beat signal of interference light from the mark WM1, and .phi..sub.N represents a phase difference between the reference signal and the beat signal of the noise signal. A phase difference between a synthesized component T.sub.S of the vector components A.sub.S and N.sub.S, and the reference signal component is represented by .phi..sub.S. Therefore, the phase difference .phi..sub.S as alignment information includes an alignment error caused by noise light, thus causing an alignment error.
In the LSA method, since a photoelectric signal is distorted due to the influence of stray light or noise light from a portion other than the alignment mark, the mark position cannot be accurately detected, thus causing an alignment shift.
As described above, only one field stop arranged in the illumination optical system cannot define a proper illumination region due to the astigmatism. The alignment sensor undesirably detects position information from a portion other than a wafer alignment mark due to the influence of stray light, thus causing an alignment error.